One known method of determining edge position and material width of a rolled material is described, inter alia, in Iron and Steel Engineer, May 1989, pp. 23-25, in an article entitled "Cold strip measurement and inspection". The method comprises using two or more video cameras directed against the rolled material. The video cameras are positioned on one and the same line across the material in such a way that the two edges are covered by the field of view of the respective outer cameras. To obtain sufficient contrasts between material and the external surroundings, either that side of the strip which faces the cameras or the under side of the material is illuminated by one or more light sources. The "image", perceived by the video cameras, of the region around the edges of the material is data processed in a known manner using conventional, modern technique such that, after calibration of the equipment, the positions of the edges can be indicated. Position data can then both be shown on a monitor and be included as measured values in an edge position or centering control device.
The environment in which the edge position measuring devices are situated may give rise to considerable problems for optical measuring methods. Keeping camera lenses and light sources free from dust and dirt often assume daily cleaning. In addition, these applications often involve high temperatures in the surroundings as well as splashing from water flushing, which may jeopardize the function of the measuring system.
Before the video technique became generally available, other measuring methods (without describing these in more detail) were available which were based on optomechanical systems as well as on sensors making direct contact with the edges of the material.
In Trosa Metaltronica AB's pamphlet S87-01, an electromagnetic method for edge position determination is described. The method comprises placing a coil under the two edges of the material. Special evaluation technique analyzes the voltage induced in the coil after a magnetizing current in the coil has been disconnected. In this way, under certain special conditions, a measure of the edge position can be obtained.
The evaluation technique used is described in SE 451 886 entitled "Method and device for contactless measurement of quantities in or near electrically conducting material". A magnetizing field is generated with the aid of a coil which can be supplied with alternating current or a pulsed polarized or unidirectional direct current. The quantities, about which information may be obtained by evaluation of the aperiodic induced voltage signal which arises in connection with the decaying magnetic field, are the distance between the coil and the electrically conducting material, the thickness of the material, and its electrical conductivity. The various quantities may be determined substantially by sampled measurement, i.e. by dividing the measurement of the induced voltage into different time intervals. The distance between the coil and the electrically conducting material is determined substantially by the magnitude of the induced voltage in a time interval immediately after switching off the magnetic field.
The voltage induced in the coils in case of a decaying magnetic field, when the coils according to the above pamphlet are placed under the material edges, will be dependent on how large a part of the coils is covered by the material. Besides this dependence, also the signal will to a very great extent be dependent on the distance between the coil and the material, as well as on a possible edge bending of the rolled material. This is one of the reasons why edge position gauges designed in this way, with edge measuring coils only, have not become more widely spread.